As illustrated in FIG. 1, a hydraulic system for a construction machine in the related art includes a variable displacement hydraulic pump 2 (hereinafter referred to as “hydraulic pump”) operated by an engine 1, a hydraulic actuator 3 (e.g., boom cylinder) operated by hydraulic fluid that is supplied from the hydraulic pump 2, a control valve (MCV) 4 installed in a flow path between the hydraulic pump 2 and the hydraulic actuator 3 to control a flow rate of the hydraulic fluid that is supplied from the hydraulic pump 2 to the hydraulic actuator 3 when a spool thereof is shifted, an oil cooler 7 installed in a return flow path 6 from the control valve 4 to a hydraulic fluid tank 5 to cool the hydraulic fluid that returns to the hydraulic fluid tank 5, a hydraulic pump regulator 8 controlling a discharge flow rate of the hydraulic pump 2 through adjustment of an inclination angle of a swash plate of the hydraulic pump 2, and a controller 9 outputting a control signal to the hydraulic pump regulator 8 or an engine revolution control device (not illustrated) of the engine 1 so as to limit the discharge flow rate of the hydraulic pump 2 to a predetermined flow rate.
The hydraulic system for a construction machine in the related art as described above operates to lower the hydraulic fluid temperature through heightening of the rotating speed of a cooling fan if the hydraulic fluid temperature of the hydraulic fluid tank 15 exceeds a predetermined upper limit value and to make the hydraulic fluid temperature belong to a certain temperature range between a lower limit value and the upper limit value through lowering of the rotating speed of the cooling fan if the hydraulic fluid temperature is lower than the predetermined lower limit value.
On the other hand, if the hydraulic fluid temperature of the hydraulic fluid is low as illustrated in FIG. 2, the viscosity of the hydraulic fluid becomes relatively high, and thus the suction pressure from the hydraulic fluid tank 5 to the hydraulic pump 2 is lowered to cause a pressure loss to occur. Due to this, in the case of abruptly operating working devices (including a boom, an arm, and a bucket) of the equipment in a state where the hydraulic fluid temperature is lower than the predetermined lower limit value, the suction pressure of the hydraulic pump 2 becomes lowered, and a cavitation phenomenon may occur. If the cavitation phenomenon occurs, the hydraulic pump 2 may be damaged or the hydraulic system of the equipment may be damaged.
Further, in the case where the atmospheric temperature is increased to an abnormally high temperature, or the hydraulic fluid temperature is excessively increased over the cooling capability of the oil cooler 7 due to trouble of the oil cooler 7 or the like, hydraulic components may be damaged, or a seal for preventing oil leak that is installed on the hydraulic actuator or hydraulic pipes may be damaged to cause oil leak.